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Filter output above or below waterline?

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Thanks for all the help earlier with my Eheim canister filter.
I introduced 9 new small fish to the tank yesterday and found 3 of the new fish dead plus another neon tetra from before dead this morning.
Tests look good except the PH was 6.6

I'm wondering if I'm doing something wrong with my filter. Should the output nozzle splash water on the surface or can it be below the waterline? I am curious if that affects the water level. It had been underwater for a couple weeks with no ill effects but now with the die off I wonder. Could that be the result of the output nozzle being underwater?
Is there a right and wrong way to do this? Thanks in advance Drew aka Santiago

Only way they would have died with output underwater vs above is if there is a low oxygen situation and the splashing was aerating the water just enough to save them. Keep in mind that low o2 is not normal and is indicative of a problem that needs fixed. No healthy tank needs more air exchange then what occurs naturally. The deaths could also be just sick fish. I need more info such as all water test resultss.

I keep my nozzle below the water line, but point up at a 45 degree angle to the surface to get a roaring rapids effect on the surface. It is much more silent this way, but I still get enough water agitation.

You can do it either way, above the water or below the water. The key is surface agitation.

Are your fish hanging near the surface? Do they look like they're gasping? What are the rest of your parameters? Has there been a big change in them recently? Did you QT the new arrivals?

If it's oxygen, one thing that might help is to run a second filter, an HOB or something that drops filtered water into the tank/penetrates the surface area. You coul set the return to at or just below the surface to help.

No I didn't QT them but they were also hanging near the surface. Also my Trapdoor snails were at the top too. All the normal tests came out good. Zero anything and 6.6 PH which may be a bit low. I added some buffer.

Stop using buffer 6.6 is perfect. Anything between 6 and 8 are quite usable as long as an acclimation using the drip method is used when adding new fish. Buff doesn't last and simply leads to more dosing. Hyacinth is fine. Your tank is oxygen starved for some reason. What is your water change schedule and when is the last time you vacumed the substrate?

Changed too much water two weeks ago. Have not vacuumed the substait for a while but I stired it up a lot when I put in some plants yesterday. Three of the fish I thought died were just hiding. The three Siamese algae eaters.
Things seem stable for now. Thanks everybody.

Was the filters cycled? Because if not, the additional bio-load of 9 fish and the ammonia produced by the mulm you stirred from the substrate might have resulted to an ammonia spike. And that's probably why everyone was hanging around/near the surface.

Adding a power head that oxygenates just on general principles. Filter recently had an impeller replaced before that the water was terrible. Progress is being made and the fish looked happy last night. Thanks!

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